For many years, the final step in producing copper of high purity has been electrolytic. Copper is commercially available on a world scale as "cathode copper", the product of this electrolytic step. Cathode copper has purity of 99.9% Cu or greater, and can be worked into products such as wire or tubing which are extremely reliable in service because of their high purity.
The most common procedure for producing cathode copper is to cast as an initial step, copper anodes from the molten output of a smelter. These anodes typically contain 95% Cu. They are gradually dissolved in cells in which the electrolyte is sulfuric acid and the cathodes are thin sheets (about 25 or 30 mils in thickness) of high purity copper. These thin "starter sheets" are also produced electrolytically and the common practice for their production is to used lead anodes in an electrolyte containing copper sulfate and free sulfuric acid and titanium cathodes. This invention relates to such titanium cathodes, sometimes referred to as "mother blanks", used for production of starter sheets.
A titanium mother blank is a sheet of high purity titanium, typically 36 inches wide by 40 inches long and 0.125 inches in thickness. It is suspended from a copper hanger bar having typical dimensions of 48".times.1.75".times.0.75." It is essential that the joint between titanium and copper be strong mechanically and also electrically and that these properties remain constant for a period of years. Cells containing titanium mother blanks are operated at 140.degree.-150.degree. F. and low current density is used to deposit very fine grained sheets of pure copper on both sides of the immersed portion of the titanium. The mother blanks are removed from the cells at the end of 24 hours and the copper deposits are removed in the form of complete sheets, 25 or 30 mils in thickness, which resemble highly flexible foils. Titanium has evolved over recent years as the preferred material for cathodes for production of starter sheets because it can be plated uniformly with coppor and the deposit can be stripped readily and completely.
Titanium is commonly welded to itself by electrical resistance welding or by fusion welding. These methods can not be used to join titanium to copper (or to most other metals) because at welding temperatures, titanium forms intermetallic compounds with copper. These brittle compounds preclude formation of a mechanically sound joint. Titanium sheets have been joined to copper hanger bars using rivets or nuts, bolts and washers. Through holes are provided in both the sheet and hanber bar and the fasteners are closed under controlled conditions. The use of fasteners has the advantage that construction of the cathode is relatively inexpensive. A disadvantage of fasteners is that one or more fasteners on a cathode may loosen in time due to daily mechanical handling of the cathode plus daily thermal cycling from 140.degree.-150.degree. F. to ambient temperature. Loose fasteners result in imperfect electrical contact between the titanium sheet and copper hanger bar and this causes uneven deposition of copper resulting in imperfect starter sheets.
Titanium sheets have also been joined to copper hanger bars without use of fasteners. This is done by first making a titanium sheathed copper hanger bar by co-extrusion of the two metals at elevated temperature. The temperature is such (400.degree. to 800.degree. C.) that the two metals deform plastically and intimate contact results but the temperature is below that at which metal-to-metal welding, accompanied by deleterious formation of inermetallic compounds, occurs. The titanium sheet is then joined to the titanium sheath of the hanger bar by conventional welding e.g., by electrical resistance welding. This adjoinment method is the subject of U.S. Pat. No. 3,857,774, which teaches that cathodes made in this manner have more stable electrical characteristics than those made with rivets or nuts and bolts. A disadvantage of co-extruded hanger bars is that they are relatively expensive due to the capital cost of the massive equipment necessary for their manufacture. A second disadvantage is that the co-extrusion process makes best use of both metals when the copper core is completely sheathed by titanium. Using January, 1979 prices, copper costs $0.75 per pound and titanium costs $6.50 per pound.
To the extent that titanium is used in any area other than that to which the titanium sheet is to be welded, the co-extrusion process is wasteful of this relatively expensive metal.